This section is intended to provide a background or context for the invention to be disclosed below. The description to follow may include concepts that could be pursued, but have not necessarily been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated below, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
In general, a dielectric filter is composed of a number of resonating structures and energy coupling structures which are arranged to exchange radio-frequency (RF) energy among themselves and input and output ports. The pattern of interconnection of these resonators to one another and to the input and output ports, the strength of these interconnections, and the resonant frequencies of the resonators determine the response of the filter.
During the design process for a dielectric filter, the arrangement of the parts, the materials from which the parts are made, and the precise dimensions of the parts are determined such that an ideal filter so composed will perform the desired filtering function. If a physical filter conforming exactly to this design could be manufactured, the filter would perform exactly as intended by the designer.
However, in practice, the precision and accuracy of manufacture of both the materials and the parts are limited, resulting in departures in the values of resonant frequencies and coupling strengths from desired values, These departures, in turn, cause the response of the dielectric filter to differ from that predicted by an ideal filter model. Often, the departures from an ideal response are sufficiently large to bring the filter outside of its design specification. Because of this, it is desirable to make use of some means for adjusting the resonator frequencies and coupling strengths to bring the filter response within the design specification.
This is particularly the case for a class of dielectric filters in which TE (transverse electric) single-mode and triple-mode ceramic-filled cavities are combined. Filters of this type are tuned by making modifications to multiple faces of the components, including faces which will be bonded together in the assembled filter. However, this prevents full tuning of a filter subsequent to bonding, because, at that time, the bonded faces are no longer accessible.
As is recognized by those of ordinary skill in the art, triple-mode cuboid resonators can be tuned by lapping controlled amounts of material from three mutually orthogonal faces of the cuboid, and subsequently resilvering those faces. This allows the frequencies of all three modes of a triple-mode cuboid resonator to be independently adjusted. Single mode slab-shaped cuboid resonators can be tuned by lapping controlled amounts of material off one or more of the narrow faces, subsequently resilvering those faces.
An alternate method to tune triple-mode cuboid resonators is to drill holes in three mutually orthogonal faces, and then either to silver the walls of the holes or to leave the holes unsilvered. This method also allows independent adjustment of all three mode frequencies. In contrast, a single-mode slab-shaped cuboid resonator can be adjusted by drilling a hole or holes into one or both of the large flat faces.
Another method is to cut slots in the silver on at least two mutually orthogonal faces. This method also allows independent adjustment of all three frequencies. A single-mode slab-shaped cuboid resonator can also be adjusted by cutting one or more slots on one or more of the narrow faces, the slots being oriented parallel to the large faces.
As noted above, however, filter components cannot be tuned after the components have been bonded together, because, after bonding, an insufficient number of faces is accessible. The present invention addresses this deficiency in the prior art.